Class AB amplifiers typically have good output, but low efficiencies. In order to improve efficiency and maintain good output, class G amplifier was introduced. The operation principle of class G amplifiers is similar to class AB amplifiers. In contrast to class AB amplifiers, the power supply of class G amplifiers is variable as the input signal varies, so that a voltage across the audio output stage is low to increase the efficiency.
FIG. 1 illustrates a prior art single-supply class AB amplifier 10. As shown in FIG. 1, the single-supply class AB amplifier 10 comprises audio output stages 11 and 12, both of which are powered by a positive power supply (VDD); speakers 13, 14, and blocking capacitors 15, 16. The blocking capacitors 15, 16 are used to block a DC bias. Typically, the DC bias is about 0.5VDD. So the capacitance of the blocking capacitors 15, 16 are large, e.g. 470 μF. The high capacitance of may interfere with external circuit, enlarge the size of the amplifier, and increase costs of manufacturing.
FIG. 2 illustrates a prior art amplifier circuit 20 that adopts a charge pump to power audio output stages. In contrast to the single-supply class AB amplifier 10 shown in FIG. 1, the amplifier circuit 20 comprises a charge pump that provides a negative power supply (VSS) to power output stages 11, 12. The negative power supply (VSS) has the same amplitude as the positive power supply (VDD), but with a different polarity. As a result, the amplifier circuit 20 does not require a high capacitance blocking capacitor. Instead, the amplifier circuit 20 includes a capacitor 21 and a fly capacitor 22 with low capacitance, e.g. 1 μF. However, when the input signal is small, the loss of the amplifier circuit 20 is high, causing low efficiencies.
FIG. 3 illustrates a prior art amplifier circuit 30 that uses two power supplies. As shown in FIG. 3, a positive power supply (HPVDD) is provided by a buck converter including a high-side switch 38, a low-side switch 39, an inductor 40, and an output capacitor 41. The duty cycles of the high-side switch 38 and the low-side switch 39 are regulated as the input signal varies through a level detector 31, an optimizer 32, an error amplifier 33, a compensation network 34, a saw-tooth wave generator 35, a PWM (pulse width modulation) comparator 36, and a driving circuit 37, so that the positive power supply (HPVDD) is regulated.
A charge pump 43 receives the positive power supply (HPVDD), and provides a negative power supply (HPVSS) which has the same amplitude to the positive power supply (HPVDD). The positive power supply (HPVDD) and the negative power supply (HPVSS) are used to power an audio output stage 42 of the amplifier circuit 30. Both power supplies of the audio output stage 42 vary as the input signal varies, reducing loss and increasing efficiency. However, the buck converter requires a large layout, even larger than a Class AB amplifier with a charge pump. The buck converter also has low efficiencies under light load. Thus, an additional inductor 40 is needed, which can increase costs and generate EMI (Electro Magnetic Interference). Accordingly, there is a need for improved class G audio amplifiers with high efficiency, small size, and low cost.